Quote from: theymos on October 02, 2010, 06:11:11 AM
If you have a lot of computers and they’re all working on the same block with the same public key, then they’re all very likely to be hashing the same block at the same time, which is pointless. To fix this, each computer is given a unique extraNonce modifier value. This might be very large to prevent collisions, and it therefore slows down hashing.
In a centralized system, the server could simply keep a list of extraNonces in active use and give out the lowest free one. Then you would avoid collisions without requiring more than one unique extraNonce per client. A 3-byte extraNonce would be sufficient for millions of clients. Note that clients of realistic performance do not really need the extraNonce to handle nonce overflow today, since such overflows happen less frequently than the periodic nTime updates (but this is not considered by the standard client today, which updates the extraNonce more liberally).